The present invention relates to a vibrating source, and particularly to a source adapted to be lowered into a relatively narrow well.
The vibrating source according to the invention can advantageously be used for the purposes of seismic prospecting, in order to explore subterranean zones likely to include hydrocarbon reserves. Seismic signals are emitted in the ground, and signals reflected by the subterranean reflectors are received by means of pickups. The received signals are recorded, and the recordings are subjected to systematic clarification treatments in order to obtain seismic sections that are accurately representative of the subsoil configuration.
Seismic prospecting can be achieved for example, by means of a vibrating source comprising a transmitting element which is in contact with the ground or the wall of a well, and means for applying sinusoidal forces to the transmitting element. The vibrations are emitted during a certain time interval, followed by a listening time interval during which the reflected waves are received. The frequency of the emitted waves can be constant, but frequency excursions can also be utilized.
To generate vibrations in the ground, vibrators of the piezoelectric or magnetostrictive type, or hydraulic or pneumatic electromagnetic vibrators are used. Different vibrators are described for example in U.S. Pat. Nos. 4,805,727, 4,774,427, 4,715,470 or 3,718,205.
French Patents 1,542,973, 1,428,395, 1,295,059 or U.S. Pat. No. 4,234,053 also notably describe vibrating sources where the motor means comprise one or several eccentric turning parts the rotation axes of which are rigidly linked to transmitting elements coupled with the ground. The developed force increases with the mass of the turning parts and with their rotation speed. Only one turning part is necessary to obtain a turning force. In order to obtain a vibrational force following a particular polarization direction, a couple made up of two parts that are rotated at an identical speed but in opposite directions, symmetrically in relation to the chosen direction, is set up. The drawback of this type of source is that the vibrational force increases with the rotation speed, therefore with the frequency of the generated vibrations. A well-known method for regulating the force consists of utilizing two couples made up of turning parts with the same rotation speed, while introducing between them an angular phase shift that is varied according to the frequency of the vibrations to be generated. However, generally speaking, angular phase shifts are obtained by mechanical means such as pinions and gears, which complicates the apparatus. Perfectly controlling the amplitude of the generated vibrational force according to the frequency is difficult to do with this type of purely mechanical compensator. Besides, it should also be noted that:
vibrators with eccentrics are generally quite bulky; they are designed for working at the surface and are not adapted for working in wells or wellbores that are most often very narrow; and
the mechanical compensators used to regulate the vibrational force according to the frequency contribute to increasing further on the volume of vibrators with eccentrics and lend themselves poorly to remote controlling.